Kidney transplantation is the preferred treatment for most end stage renal diseases, but remains encumbered by its requirement for chronic, multi-drug immunosuppression to prevent rejection. These regimens are incompletely effective ? most recipients eventually lose graft function to indolent rejection ? and are associated with serious side effects. Two particular agents, calcineurin inhibitors (CNIs) and steroids are widely used and dominate the morbidity observed in most patients. In 2011, belatacept, a fusion protein specific for CD28/B7 co-stimulation pathways, was introduced as a CNI alternative. The approved belatacept regimen, although avoiding CNIs, relies on steroids, and more importantly, has been associated with high rates of early acute rejection. Over the past 6 years, we have conducted a phase 2 clinical trial under the FDA OPD program investigating an alternative belatacept-based regimen that combines three approved drugs: alemtuzumab, belatacept and sirolimus, avoiding both CNIs and steroids, and reducing the rate of early acute rejection. The initial trial has been highly successful. Enrollment has been completed in two cohorts; the first, 20 low immunologic risk recipients of living related donor kidneys, and the second, a more generalizable cohort of 20 recipients, including deceased donor kidneys and sensitized recipients. There has been no graft or patient loss, and the regimen has avoided early rejection. Importantly, we have shown that many patients can be eventually withdrawn from sirolimus and maintained solely on belatacept, avoiding all daily immunosuppressive drugs in lieu of a single monthly infusion free of off-target side effects. As the funding cycle of the OPD grant mechanism is shorter than the time required to complete follow-up of enrolled patients, this application seeks three years of funding to complete the trial, and glean its unique clinical and mechanistic insights. The proposed study has two Specific Aims, each stemming from the ongoing study. Specific Aim 1 facilitates clinical follow-up of all patients transplanted under this trial to their planned 5-year post-transplant time point, continuing their medical and immunosuppressive management, study-related biopsies, and functional assessment in keeping with the originally approved protocol. Specific Aim 2 allows us to study this unprecedented cohort of patients, contrasting those maintained rejection-free on belatacept monotherapy to those requiring adjuvant immunosuppression. Specific attention will be directed toward polychromatic, flow cytometric immunophenotyping, through the period of homeostatic repopulation and into the establishment of a post-transplant steady state, to characterize the nature of patients maintained rejection-free solely through blockade of the CD28/B7 pathway. We will follow-up and develop observations made during the initial course of this study. These include post-depletional reconstitution of a more nave T cell repertoire with enhanced reliance on CD28 co-stimulation, action of regulatory lymphocyte populations whose function is unfettered without CNIs, and sirolimus-driven endothelial changes fostering a tolerogenic graft-recipient interface.